The purpose of this project is to study and compare the recognition and rejection of allografts of leukemia with those of normal hemopoietic cells and of other tissues. The information obtained will be correlated with known histocompatibility differences by using congenic-resistant lines of mice for graft exchanges. The influence of alleles at selected gene loci on susceptibility to syngeneic and allogeneic lymphomas, sarcomas, and carcinomas will also be investigated. These genetic loci influence susceptibility to normal heompoietic isografts, allografts, and xenografts. Attempts will be made to identify cell populations responsible for rejection of particular kinds of grafts (hemopoietic versus non-hemopoietic, normal versus neoplastic) or in particular experimental conditions (irradiated versus non-irradiated animals, allogeneic versus parent-to-hybrid strain combinations). Studies of target cell killing in vitro by lymphoid cells are contemplated to confirm the interpretation of graft rejection experiments carried out in vivo.